Chloritoid composition and formation in the eastern Central Alps : a comparison between Penninic and Helvetic occurrences Autor(en): Rahn, Meinert K. / Steinmann, Marc / Frey, Martin Objekttyp: Article Zeitschrift: Schweizerische mineralogische und petrographische Mitteilungen = Bulletin suisse de minéralogie et pétrographie Band (Jahr): 82 (2002) Heft 2: Diagenesis and Low-Grade Metamorphism PDF erstellt am: 11.10.2021 Persistenter Link: http://doi.org/10.5169/seals-62373 Nutzungsbedingungen Die ETH-Bibliothek ist Anbieterin der digitalisierten Zeitschriften. Sie besitzt keine Urheberrechte an den Inhalten der Zeitschriften. Die Rechte liegen in der Regel bei den Herausgebern. Die auf der Plattform e-periodica veröffentlichten Dokumente stehen für nicht-kommerzielle Zwecke in Lehre und Forschung sowie für die private Nutzung frei zur Verfügung. 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MITT. 82, 409-426,2002 Chloritoid composition and formation in the eastern Central Alps: a comparison between Penninic and Helvetic occurrences by Meinert K. Rahn'- *, Marc Steinmann2 and Martin Frey3(deceased) Abstract Chloritoid occurs in Middle Cretaceous shales (Bündnerschiefer) of the Tomül nappe in the Safien valley. Compositional and textural data of chloritoid are compared with nearby occurrences at Curaglia/Mutschnengia (sedimentary cover of the Gotthard nappe) and at Kunkels Pass (Helvetic Autochthonous). At the Tomül nappe profile, the observed mineral assemblage is phengitic mica-chlorite-quartz-chloritoid-paragonite-mixed layer phengite/paragonite. Textures indicate syn- to post-kinematic growth of chloritoid with respect to the main schistosity. Temperatures derived from chloritoid-chlorite and chlorite thermometry suggest metamorphic peak conditions of 400 ± 50 °C pre- D2, i.e. before Early Oligocene, but these have to be considered with caution due to high Mn contents in chloritoid. Comparison with neighbouring occurrences indicates very similar, but more Mn-depleted chloritoid compositions at Kunkels Pass and Curaglia. In these localities, chloritoid forms rosettes growing across the main schistosity, which indicates post-kinematic growth after the Early Oligocene. Whereas temperatures obtained for Curaglia are similar to those for the occurrence in the Tomül nappe, they are distinctly lower (300-350 °C) for Kunkels Pass, where chlorite does not coexist with chloritoid. The formation of chloritoid is discussed for various bulk rock compositions on the basis of a four-component multisystem. The Al contents, the amount of carbonate, and a value of XMg < 0.45 turn out to be important factors. The formation of chloritoid in Bündnerschiefer is assumed to be due to a reaction muscovite + chlorite —> chloritoid + celadonite; alternatively, its formation at the expense of high-pressure ferrocarpholite may be suggested. However, unlike the observations in other parts of the North Penninic Bündnerschiefer, no observations in favour of the formation of chloritoid at high pressures, due to ferrocarpholite breakdown, have been made in the Tomül profile studied. Keywords: Chloritoid, chlorite thermometry, North Penninic, Infrapenninic, Helvetic. 1. Introduction metamorphic grade compared to the Helvetic units further north (Frey and Ferreiro Mähl- Chloritoid (eld) appears as an important index mann, 1999). However, only few reports of eld mineral in Al-rich shales at the onset of the green- have been published from this area so far (Niggli schist faciès (Bucher and Frey, 1994). In the and Niggli, 1965; Oberhänsli et al., 1995). Central Alps, the reaction pyrophyllite (prl) + The presence of typical high-pressure (HP) chlorite (chl) —> chloritoid (eld) + quartz (qtz) + minerals such as ferrocarpholite, glaucophane H,0 was mapped as a cld-in isograd by Frey and and lawsonite within the sediments of the North Wieland (1975). In the area they investigated, Penninic palaeogeographic belt (Oberhänsli et located in the eastern Central Alps, this reaction al.. 1995; Goffé and Bousquet, 1997; Bousquet isograd roughly follows the southern end of the et al., 1998) raises interesting questions about the Helvetic belt (Fig. 1), suggesting that eld should metamorphic history of the Helvetic and adjacent be common in Al-rich Penninic Bündnerschiefer North Penninic units, and about the continuity of further south, because of their slightly higher the metamorphic pattern across the Penninic 1 Institut für Mineralogie, Petrologie und Geochemie, Albert-Ludwigs-Universität, Albertstrasse 23b, D-79104 Freiburg, Germany. * Present address: HSK, CH-5232 Villigen-HSK, Switzerland. <[email protected]> 2 Département de Géosciences, UFR Sciences et Techniques, Université de Franche-Comté, 16 route de Gray, F-25030 Besançon cedex, France. <[email protected]> 1 Mineralogisch-Petrographisches Institut, Universität Basel, Bernoullistrasse 30, CH-4056 Basel, Switzerland. 410 M.K. RAHN. M. STEINMANN AND M. FREY thrust front, as suggested by the latest compilation part of a post-kinematic metamorphic overprint, of metamorphic data (Frey et al., 1999). eld within North Penninic units was proposed to While HP indicators have been reported from result from the breakdown of ferrocarpholite at North Penninic units within the Engadine pressures above 0.7 GPa (Goffé and Bousquet, window (Bousquet et al., 1998), and from the Peiden 1997; Bousquet et al., 1998). The purpose of this slice (Oberhänsli et al., 1995) north of the Adula paper is a detailed comparison of compositional nappe (Fig. l),such evidence is absent in the and textural aspects of chloritoid occurring in Helvetic belt. This fact is attributed to a Late different tectonic units of the eastern Central Alps. Cretaceous/Early Tertiary subduction event affecting The question of how eld is formed is discussed on the sediments and subordinate magmatic rocks of the basis of bulk rock and mineral compositions the former Valais trough subsequent to their on one side, and textural aspects on the other. incorporation into the Alpine orogeny, while sedimentation was ongoing in the Helvetic realm (Pfiffner, 1986). 2. Geologic Setting and Sample Localities While Frey (1969), Frey and Wieland (1975) and Mayerat (1986) have discussed the presence The Central Alps are characterised by a relatively of eld within Helvetic and Infrapenninic units as strict southwards and upwards succession of tectonic units, which represents the result of a Tertiary continent-continent collision subsequent to Early Mesozoic rifting and Late Cretaceous to Tertiary convergence. During rifting two oceanic realms were formed, and later closed by convergence. The two colliding continental plates, the Eurasian plate in the N (the present-day Helvetic belt), and the Apulian microplate in the S with the present-day Austroalpine/Southalpine units represent the lowermost and uppermost units, respectively, of the actual Alpine edifice. The Penninic units in-between are remnants of an extended southern ocean (Piemont-Liguria), an intermediate "terrane" (Stampfli and Marchant, 1997) or crystalline rise (Briançonnais), and a northern ocean (Valais trough) of debated size and extension (Stampfli and Marchant, 1997; Oberhänsli, 1994, Steinmann and Stille, 1999). In the actual Alpine edifice, these form the South, Middle, and North Penninic units, respectively. Austroalpine basement Sample localities: According to their palaeogeographic origin and Austroalpine sediments (l) Tomül nappe profile time of incorporation into the Alpine orogeny, Penninic basement (2) Grava nappe profile each of these main units have undergone distinct in I North Penninic sediments (J) Curaglia/Mutschnengia structural and metamorphic histories, which can be inferred from the metamorphic A: Aul slice, G: Grava nappe (J) Runkels Pass part pattern at the present surface (Frey et al., 1999). S: T: Tomül Schams nappe, nappe The South Penninic units underwent shortening Helvetic/lnfrapenninic basement and metamorphism since the beginning of the £ Helvetic (par)autochthonous cover } Helvetic belt closure of the Tethys ocean some 100 Ma ago ] Helvetic/lnfrapenninic allochthonous cover (Stampfli et al., 1998). It is assumed that a large chloritoid-in reaction isograd portion of the South Penninic oceanic substratum was subducted during convergence. Remnants of this oceanic crust are represented by ophiolites Fig. 1 Tectonic map of the eastern Central Alps, modified (Zermatt-Saas Fee, Aosta ophiolites, Platta after Trümpy (1980) and Steinmann (1994a,b).The nappe, Arosa zone, Ramosch slice, see Bearth, three localities to three different palaeo- sample belong 1967; Trümpy, 1988), and serpentinites (Malenco, tectonic units,